1. Field of the Invention
The present invention relates to a thin film single crystal substrate which is useful in the production of a semiconductor. More particularly, it relates to a thin film single crystal substrate comprising a single crystal diamond base and at least one film of a single crystal material formed thereon.
2. Description of the Prior Art
Some of materials having a crystalline structure is useful as materials of electric elements. For example, silicon carbide has been studied as a material for a heat and environment resistant element or a light-emitting element since it has several crystal structures and band gap of 2.2 to 3.3 eV depending on the crystal structure, its heat conductivity is as large as 4.9 W/cm.K, its maximum electron mobility is confirmed to be 1,000 cm.sup.2 /V.sec., and it is possible to control p-type and n-type valence electrons.
The crystal structures of silicon carbide are roughly classified into an .alpha.-type and a .beta.-type. Since silicon carbide of the .alpha.-type hexagonal system (6H) has a large band gap of about 2.9 eV, it is believed that it can be used as a material for a blue light-emitting element. Silicon carbide of the .beta.-type cubic system (3C) has a large electron mobility, it is investigated as a material for an environment resistant element.
Since a silicon carbide single crystal has a high melting point and is chemically stable, it is difficult to produce a large single crystal with good quality which can be used as a semiconductive material. Therefore, the single crystal of silicon carbide is formed by growing it on a silicon single crystal or sapphire in a gas or liquid phase (cf. Japanese Patent Kokai Publication Nos. 83588/1978 and 146299/1978).
However, the silicon single crystal has various drawbacks. For example, since it has a band gap of 1.1 eV which is smaller than that of the silicon carbide single crystal, it has smaller resistivity at high temperature. It has a low melting point and a small thermal conductivity of 1.5 W/cm.K). Because of these characteristics of silicon carbide, when the silicon carbide single crystal is grown on the silicon single crystal, the advantages of the latter as the heat resistant semiconductive material are greatly impaired. Therefore, the silicon carbide single crystal grown on the silicon single crystal is used by removing the latter with an acid, but the remaining thin film of the silicon carbide has a thickness of several ten .mu.m and is easily cracked or strained, which deteriorates its handling properties.
When the silicon carbide single crystal is used without removing the silicon single crystal, elements should be separated by applying reverse bias at a p-n junction formed in the silicon carbide layer. This makes the production method complicated.
Although sapphire is an insulating material up to high temperature and thermally stable, it has some drawbacks such as a small coefficient of thermal conductivity of 0.11 W/cm.K and a large coefficient of thermal expansion of 7.8.times.10.sup.-6 /K.
In addition to the above proposals, it may be possible to grow a thin film single crystal of GaAs on the silicon single crystal or to grow the silicon single crystal on sapphire. However, these techniques have the same problems as above.